1. Field of the Invention
This invention relates to a rotating radiation chamber for radiation therapy which has a radiation beam irradiating section rotatable round a patient, particularly to a rotating radiation chamber for radiation therapy which is suitably used as a rotation gantry of a cancer therapy device by use of a proton beam.
2. Description of the Related Art
Conventional cancer therapy based on radiation of active rays uses X-rays, gamma rays, electron beams, fast neutron beams, etc. These active rays, as shown in FIG. 9, become the strongest at sites close to the surface of a patient, and thus may inflict damages on normal tissues close to the body surface when those rays are directed towards a cancer in a deeper part of the body. By the way, a proton or a particle which comes into being when a hydrogen atom has been removed of the electron, has a positive charge, and has a mass 1836 times as large as that of electron, can be accelerated under a high energy state by an accelerator to give a proton beam. The proton beam is characterized by having the maximum dose peak or a Bragg peak P at a certain depth from the body surface, and then declining rapidly to zero.
This is because, as the electric force a proton exerts on electrons becomes large in proportion to its proximity to the latter, when the proton has a high kinetic energy and runs at a high speed, the time for the proton to interact with nearby electrons is short, and ionization is small in magnitude, but, when it loses the kinetic energy to nearly make a stop, the time for interaction becomes long and ionization rapidly increases in magnitude.
Thanks to this nature peculiar to protons, it is possible to apply proton beams for cancer therapy keeping normal cells other than a cancer comparatively free from damages, even if the cancer lies in a deeper part of the body. Further, as the radiation-based biological effect (RBE) of a proton beam is nearly equal to that of X-rays, the proton radiation therapy is advantageous in that it can make the most of knowledge and experience accumulated in the field of conventional X-ray radiation therapy. With these features, the proton radiation therapy device is being introduced as a therapy means to treat a cancer without removing any functional organs and encroaching on the quality of life.
In the radiation therapy of cancer, it is ideal to concentrate a lethal dose of active rays onto the cancer alone without inflicting any irreversible damages to nearby normal tissues. The Proton radiation therapy, as shown in FIG. 9, exploits the feature characteristic with protons that a proton beam incident on a substance gives the maximum dose or Bragg peak P just before it ceases to move. Namely the therapy in question aims at achieving this ideal by covering only the cancerous lesion with that Bragg peak.
By the way, protons obtained from an accelerator are in the form of a slender beam, and its energy is constant (the depth of Bragg peak is also constant). On the other hand, cancerous lesions are varied in size and have complex shapes, and their depths in the body are not constant. Further, the density of tissues through which a proton beam must pass is not constant neither. Accordingly, to achieve an effective radiation therapy, it is necessary to (1) enlarge the proton beam to have a sufficient width to cover the whole cancer lesion in one radiation; (2) adjust the beam energy according to the depth of lesion; (3) give a sufficient energy distribution in depth so that the whole cancer lesion having a certain depth can receive a uniform irradiation; and (4) make corrections according to the irregularities in contour of the lesion, and in density of the tissues through which the proton beam must pass.
It is also necessary to accurately focus the proton beam which has been adjusted in accordance with the shape and depth of a tumor, onto the cancerous lesion in the body of a patient following the radiation condition as determined previously, and to adjust radiation so that its dose and distribution occur as designed, and fall within tolerable errors.
To achieve above, it is necessary not only to reproduce the initially designed distribution of dose by properly adjusting the irradiation field forming unit including a bolus and a collimator, but also to accurately determine the radiation position of a proton beam with respect to the patient.
To attain this object, a rotating radiation chambers for radiation therapy has been developed which has a radiation beam irradiating section rotatable round the patient.
However, as shown in FIGS. 10 to 12, the conventional rotating radiation chamber is so constructed as to push forward a projecting deck 36 as needed, in synchrony with a radiation beam irradiating section 22, together with a treatment bed 32 which is also driven forward from the housing base 30 by a bed driving mechanism 34, into a rotating capsule 20 which rotates round a patient 10 as indicated by the arrow of FIG. 12. Thus, when the radiation beam irradiating section 22 rotates until it comes beneath the treatment bed 32 to allow the radiation beam 24 to strike the patient 10 from a proper direction, the radiation beam irradiating section 22 and the projecting deck 36 interfere each other in their paths. Accordingly, a deck driving mechanism 38 and its control section must be added to project or retreat the projecting deck 36, in accordance with the position of radiation beam irradiating section 22. This complicates the structure. Further, as the deck driving mechanism 38 has to be installed on the housing base 30 on the same side with the rotating capsule 20, and on the same side the bed driving mechanism 34 is also installed, the deck driving mechanism 38 might interfere with the bed driving mechanism 34 but for some preventive means against it. Avoiding such interference with the bed driving mechanism 34 would further complicate the structure of deck driving mechanism 38. Furthermore, when the radiation beam irradiating section 22 comes downwards, and the projecting deck 36 is retreated, access to the interior of rotating capsule 20 becomes impossible. Still further, as the projecting deck 36 and the rotating capsule 20 are entities independent of each other, they have inevitably a space between them, through which a hard object may fall down, or their existence limits the work space. Still further, although a back panel 26 is installed against the background within the rotating capsule 20 to form a part of the enclosure surrounding the patient 10, but the floor is open. Therefore when the projecting deck 36 is retreated, the distance from the treatment bed 32 to the floor to be lengthened because then the inner surface of rotating capsule 20, instead of the projecting deck 36, forms the floor, which may cause a fear in the patient 10.